Imaging elements, particularly photographic silver halide imaging elements, commonly use a hydrophilic colloid as a film forming binder for layers thereof. The binder of choice in most cases is gelatin, prepared from various sources of collagen, most commonly osseine (see, e.g., P. I. Rose, The Theory of Photographic Process, 4th Edition, edited by T. H. James (Macmillan Publishing Company, New York, 1977) p. 51-65). The binder is expected to provide several functions, primarily to provide an element with some level of mechanical integrity and contain all the materials within the imaging element, which are required to provide an image. In particular, in photographic elements, the binder is expected to facilitate the diffusion of materials into and out of the element during a wet processing step. Gelatin is particularly suitable to perform this function, since it can absorb water and swell during the processing steps. In addition, gelatin also forms a cross linked network below a critical setting temperature through hydrogen bonding, which prevents dissolution of the gelatin when wet. However, most photoprocessing operations are carried out above the critical temperature, which would thereby melt the gelatin in a non-crosslinked form. In order to prevent the dissolution of the gelatin during the photoprocessing operation, the gelatin is crosslinked chemically, with a hardener, during the manufacture of the imaging element.
Imaging elements using gelatin as the binder are typically prepared by first dissolving gelatin in water. Other photographically useful materials may be added to the aqueous gelatin solution to complete the aqueous coating solution. These aqueous coating solutions are then coated on a support, as single or multiple layers, coated simultaneously or in sequence. The aqueous gelatin layers are dried in a drying section of the coating machine. Rose notes in the aforementioned reference that gelatin layers swell in water upon processing, and that stresses associated with the swelling process must be relieved. Because the layers are bound to the support, vertical swelling is the most important mechanism for relieving these stresses, since the layers are not free to swell laterally. Depending upon the conditions used to dry the gelatin layers of the imaging element, large lateral stresses can be induced, which upon processing, can result in buckling of the layers. This buckling occurs in an irregular pattern known as reticulation (P. I. Rose, The Theory of Photographic Process, 4th Edition, edited by T. H. James (Macmillan Publishing Company, New York, 1977) p. 62-63). In the final image, reticulation is manifested as lower gloss and higher haze in the final processed image, decreasing the commercial value.
High purity gelatins are generally required for imaging applications. Gelatins are made from sources of collagen. The collagen may be obtained from many sources known in the art, such as bones and hides. Bovine bones and pig skins are most commonly used.
The most commonly employed manufacturing process for obtaining high purity gelatins involves demineralization of a collagen containing material, typically cattle bone. The demineralized bone is known as osseine. This step is then followed by extended alkaline treatment (liming) and finally gelatin is extracted with water of increasing temperature as described in U.S. Pat. Nos. 3,514,518 and 4,824,939. The gelatin produced by this process, commonly referred to as lime processed osseine gelatin, has existed with various modifications throughout the gelatin industry for a number of years. The liming step of this process requires up to 60 days or more, the longest step in the approximately 3 month process of producing gelatin. The hydrolyzed collagen is extracted in a series of steps to obtain several gelatin fractions with varying molecular weights. In order to obtain gelatin of desired molecular weight to provide suitable coating solution viscosities, these fractions can be further hydrolyzed by high temperature hydrolysis. The fractions are then blended to obtain the appropriate molecular weight for photographic use.
Due to the length of time required to lime-process, acid-treatment of osseine may alternatively be employed. In the manufacture of acid processed osseine (APO), extractions begin immediately after demineralization and removal of excess acid, omitting the liming step. The gelatin is extracted in water at an acidic pH, in a series of fractions obtained at increasing temperatures. The acid processing of gelatin coincides with the lime processing of gelatin, except with respect to the liming step. The time required to prepare the osseine for gelatin extraction is reduced to about three days. Gelatins produced from acid-treatment exhibit different properties from lime-processed gelatins, especially the isoelectric point and gel strength.
The physical properties of gelatin, such as the isoelectric point (pI), which is the pH at which the gelatin exhibits a neutral charge, and gel strength or bloom, which is the weight in grams required to depress a plunger of 0.5 inch diameter (1.27 cm), with a {fraction (1/64)}th inch (0.38 cm) radius of curvature at the bottom by 4 mm measured for a 6.16% dry weight gelatin after 24 hours hold at 10.0° C., depend upon the nature of the processing, such as lime or acid, as discussed above. The liming process results in extensive alkaline deamidation of the amides, glutamine and asparagine, to the corresponding acids, glutamic and aspartic acid, increasing the net negative charge on the protein. It has generally been noted that the pI of lime processed osseine (LPO) gelatin is typically in the range of approximately pH 4.7-5.3. Acid processed osseine (APO) gelatins typically exhibit higher pI values than lime processed gelatins. Acid processed cattle bones are typically in the range 6.0-8.5, while acid-processed pigskin (APP) gelatin is typically much higher, at around pH 9. While the use of acid processed gelatins in photographic elements offer a cost advantage, they may lead to undesirable photographic element layer coating properties.
The use of acid-processed gelatins in the uppermost layers of a photographic element can reduce the tendency to reticulation, as in U.S. Pat. No. 4,146,398. While an acid-processed gelatin is useful in a color photographic material, the use of acid-processed pigskin is undesirable due to the tendency to form coascervates, or slugs, with lime-processed gelatins. Instead, it is typical to use acid-processed bovine bone gelatins with isoelectric points of about 6.7-7.0. It has been observed that bovine acid-processed gelatins with even higher isoelectric points, in the range of 7.5-8.5, can provide even better resistance to reticulation, but results in the deterioration of other properties of interest.
One property, which suffers from the use of bovine acid-processed gelatins, is the tendency to viscosity increases with time in coating solutions. U.S. Pat. No. 5,998,120 discloses that solutions of pure APO gelatins in concentrated dispersions of photographically useful compounds leads to viscosity increases with time at standard operating temperatures. It has been found that this tendency is a property of acid-processed gelatins in general, and this tendency toward viscosity increases with time can be observed in coating solutions as well, which are typically more dilute and lower in viscosity than dispersions of photographically useful materials as described in 5,998,120. This tendency can complicate manufacturing conditions, such as requiring dilution of the gelatin containing coating solution or increased operating temperatures of the coating solutions. Such practices may result in undesired increased wet load, lower throughput, coating nonuniformity and chemical instability. The use of acid-processed gelatins with high isoelectric points, in the range of 7.5-8.5, can exacerbate this problem.
More recently, concerns about bovine spongiform encephalopathy (BSE or “Mad Cow Disease”) have resulted in a reduced supply of cattle bone for producing both lime and acid-processed gelatins. Subsequent regulations on the production of gelatins for human consumption have created a need for new sources of gelatin for the production of imaging materials.